Modern mobile communication technology tends to provide high-data-rate multimedia services for users. FIG. 1 is a schematic diagram illustrating the structure of a System Architecture Evolution (SAE).
In FIG. 1, User Equipment (UE) 101 is a terminal device for receiving data. An Evolved Universal Terrestrial Radio Access Network (E-UTRAN) 102 is a radio access network which includes an eNodeB/NodeB for providing a radio network interface for the UE. A Mobile Management Entity (MME) 103 is configured to manage mobility contexts, session contexts and security information of the UE. A Serving Gateway (SGW) 104 is configured to provide functions of a subscriber plane. The MME 103 and the SGW 104 may be located in the same physical entity. A Packet Gateway (PGW) 105 is configured to implement charging and legal monitoring functions. The PGW 105 and the SGW 104 may be located in the same physical entity. A Policy and Charging Rules Function (PCRF) 106 is configured to provide QoS policies and charging rules. A Service GPRS Supporting Node (SGSN) 108 is a network node device for providing routing for data transmission in a Universal Mobile Telecommunications System (UMTS). A Home Subscriber Server (HSS) 109 is a home subsystem of the UE and is configured to protect user information including the current location of the UE, the address of a serving node, user security information and packet data contexts of the UE.
In current Long Term Evolution (LTE) system, each cell supports bandwidth of 20 MHz at most. In order to increase the peak rate of UE, a Carrier Aggregation (CA) technology is introduced in a LTE-Advanced system. Through the CA technology, the UE may communicate with multiple cells that are managed by the same evolved Node B (eNB) and work on different frequencies, which makes transmission bandwidth up to 100 MHz and doubles the uplink and downlink peak rate of the UE.
In order to increase the transmission bandwidth, one user may be served by multiple cells, and these cells may be located in one eNB or multiple eNBs, and thus this technology is called CA technology. FIG. 2 is a schematic diagram illustrating cross-eNB CA. For UE working in the case of CA, aggregated cells include a Primary Cell (PCell) and a Secondary Cell (SCell). There is only one PCell, and the PCell is a serving cell and is always in an active state. The PCell can be handed over only through a handover process. The UE transmits and receives Non-Access Stratum (NAS) information only in the PCell, and a Physical Uplink Control Channel (PUCCH) is transmitted only in the PCell.
In a current system, the MME needs to encrypt signalings of non-access layer and the eNB needs to encrypt Radio Resource Control (RRC) signalings and data. The MME calculates an initial key (KeNB) and transmits the KeNB to the eNB. Further, the MME transmits a set of parameters (Next Hop, called NH for short, and Next Hop Counter, called NCC for short) to the eNB through a handover message. These parameters may be used for calculating the KeNB. In an X2 handover process, a source eNB calculates a KeNB used by a destination eNB. The KeNB used by the destination eNB may be a new KeNB that is generated based on a KeNB currently used and also referring to the downlink frequency and Protocol Control Information (PCI) of the destination eNB. This method is called a horizontal generating method. The KeNB used by the destination eNB may also be generated according to the NH and NCC saved by the source eNB. This method is called a vertical generating method. These methods have been defined in a current protocol 3GPP 33.401. The UE generates its KeNB by using the same algorithm as that used by the MME. When the UE performs X2 or S1 handover, the eNB transmits a message to the UE, to inform the UE of the NCC used currently by the eNB and indicate the UE to generate a new KeNB. The UE generates the new KeNB according to the horizontal or vertical generating method. The NCC is used to determine which one of the horizontal generating method and the vertical generating method is used by the UE. If the NCC is the same as the NCC corresponding to the KeNB used currently by the UE, the UE uses the horizontal generating method, and if the NCC is different from the NCC corresponding to the KeNB used currently by the UE, the UE uses the vertical generating method.